Engineering of the protein environment around the redox‐active TyrZ in photosystem II

The photosystem II reaction centre protein D1 is encoded by the psbA gene. By activation of the silent and divergent psbA1 gene in the cyanobacterium Synechocystis 6803, a novel D1 protein, D1′, was produced [Salih, G. & Jansson, C. (1997) Plant Cell 9 , 869–878]. The D1′ protein was found to be fully operational although it deviates from the normal D1 protein in 54 out of 360 amino acids. Two notable amino‐acid substitutions in D1′ are the replacements of F186 by a leucine and P162 by a serine. The F186 and P162 positions are located in the vicinity of the reaction centre chlorophyll dimer P680 and the redox‐active Y161 (TyrZ), and F186 has been implicated in the electron transfer between Y161 and P680. The importance of F186 was addressed by construction of engineered D1 proteins in Synechocystis 6803. F186 was replaced by leucine, serine, alanine, tyrosine or tryptophan. Only the leucine replacement yielded a functional D1 protein. Other substitutions did not support photoautotrophic growth and the corresponding mutants showed no or very poor oxygen evolving activity. In the F186Y and F186W mutants, the D1 protein failed to accumulate to appreciable levels in the thylakoid membrane. The F186S mutation severely increased the light sensitivity of the D1 protein, as indicated by the presence of a 16‐kDa proteolytic degradation product. We conclude that the hydrophobicity and van der Waals volume are the most important features of the residue at position 186. Exchanging P162 for a serine yielded no observable phenotype.